The present invention generally relates to a voltage measuring device and more particularly, to an electrostatic voltage sensor compact in size at high accuracy, and having a large voltage measuring range.
For a voltmeter, there have commonly been required various conditions such as a high input impedance, a favorable stability against temperature variation and during operation, a high resistance against noises, a sufficient withstand voltage characteristic, and a high reading accuracy, etc.
Conventionally, a vacuum tube type voltmeter has been widely employed as a voltmeter having a high input impedance. However, the known vacuum tube type voltmeter as described above has disadvantages in that, due to the employment of vacuum tubes, characteristics thereof are rather unstable, with consequent large heat noises and difficulties in the reduction of its size. Accordingly, it has been a recent trend that voltmeters employing field effect transistors (FET) are gradually taking the place of the vacuum tube type voltmeters, owing to such advantages that the former have input impedance generally equal to the vacuum tube type voltmeters with less heat noises, and can be formed into a compact size. The known voltmeters employing field effect transistors, however, still have drawbacks in that, since withstand voltage characteristics of the field effect transistors are not sufficient, the voltmeters employing such field effect transistors are extremely weak against excessively large voltages, normally withstanding voltages of only about several tens of volts or thereabout.
On the other hand, in electronic equipment and appliances currently in use, it is not very uncommon that voltages in the order of several hundred to several thousand volts are utilized, and thus, the voltmeters employing the field effect transistors have been unsuitable for applications to measurements of circuit voltages for the electronic equipment and appliances as described above.